Soliton communication can overcome most of the system limitations imposed by fiber nonlinearities and dispersion for high bit rates (&gt;10 Gbps) over long distances (&gt;100 km). One residual effect is polarization mode dispersion (PMD), described by the parameter D.sub.P (in ps km.sup.-1/2) resulting, for solitons, in dispersive wave radiation, pulse reshaping and broadening, increased interaction of adjacent solitons, and timing jitter. This has been discussed in the art, for example by S. G. Evangelides, L. F. Mollenauer, J. P. Gordon and N. S. Bergano, "Polarization multiplexing with solitons," J. Lightwave Technol. 10(1), 28-35 (1992), L. F. Mollenauer, K. Smith and J. P. Gordon, "Resistance of solitons to the effects of polarization dispersion in optical fiber," Opt. Letts. 14(21),1219 (1989), L. F. Mollenhauer and J. P. Gordon, "Birefringence-mediated timing jitter in soliton transmission," Opt. Letts. 19(6), 375-377 (1994), P. K. A. Wai, C. R. Menyuk and H. H. Chen, "Effects of randomly varying birefringence on soliton interactions in optical fibers," Opt. Letts. 16(22), 1735-1737 (1991), and P. K. A. Wai, C. R. Menyuk and H. H. Chen, "Stability of solitons in randomly varying birefringent fibers," Opt. Letts. 16(16), 1231-1233 (1991).
These problems are greatly reduced if D.sub.P &lt;0.3 D, where D is the group velocity dispersion, as discussed by Mollenauer, Smith and Gordon, "Resistance of solitons to the effects of polarization dispersion in optical fiber. " Polarization of the data is also an issue for polarization division multiplexing (PDM). PDM can effectively double the data rate by using orthogonally polarized data channels which is possible with solitons because they retain a single state of polarization over transoceanic distances even in the presence of amplified spontaneous emission noise. On the other hand, PDM is not possible with linear, "non-return to zero" (NRZ) systems because of the depolarizing effects of random birefringence, as discussed by Evangelides et al., "Polarization multiplexing with solitons. "
It was unknown until the present invention that replacing the step index or dispersion-shifted fiber of standard fiber ring or figure eight laser configurations with circularly birefringent fiber could produce the desired twisted solitons. As an added advantage, the control of polarization in the birefringent laser cavity should improve its performance over polarization insensitive cavities where small changes in birefringence (externally-induced) can lead to dramatic changes in the output of the laser, as discussed by A. J. Stentz and R. W. Boyd, "Effects of polarization on mode locking in fiber figure-eight lasers," Opt. Letts. 19(18), 1462-1464 (1994).
One object of the present invention is to construct an optical amplifier that can be incorporated within a fiber laser for generating circularly polarized, twisted solitons that could easily couple to circularly polarizing transmission fiber or other fiber.
Another object of the present invention is to achieve control of polarization in an optical amplifier or fiber laser's birefringent laser cavity, in order to improve its performance over polarization insensitive cavities, in which small, extemally-induced changes in birefringence can lead to dramatic changes in the output of the laser.
Still another object of the present invention is to achieve simultaneous generation of orthogonal, twisted solitons from an optical amplifier or single fiber laser source.
These and other objects are achieved by the present invention, which comprises a method and associated apparatus for using circularly birefringent fiber for generating circularly polarized, twisted solitons that could easily couple to circularly polarizing transmission fiber. Example embodiments for use of the invention, including a ring laser, figure eight laser, and nonlinear optical loop mirror, are taught.